In the course of research in the field of autostereoscopic display, a great number of methods and arrangements have been developed, which give impressions of space to one or several viewers. These arrangements, however, frequently allow but a limited presentation of common text or two-dimensional graphs, which is the case, e.g., with U.S. Pat. No. 5,457,574 and U.S. Pat. No. 5,606,455. For users it is of advantage, though, if they can switch between 3D display not requiring special eyeglasses and high-resolution, least-impaired 2D display on one and the same device.
Means used for the optical display of the perspective views of an object in autostereoscopic presentation include, among others, electronically controlled color LCD panels, which, when controlled in the conventional way, are suitable also for two-dimensional image display. In many applications there is a keen interest to have a facility for switching from the spatial, autostereoscopic display (hereinafter also called three-dimensional display because of the strong impression of space it conveys) into a two-dimensional display. This is relevant especially for the readability of text, as the image quality is higher in the two-dimensional mode, due to the higher resolution of the image.
A number of arrangements are known that allow such switching over from 2D to 3D and vice versa. In WO 01/56265, for example, a method for spatial display is described in which at least one wavelength filter array provides 3D display. In a special embodiment of this invention, an LCD panel acts as a wavelength filter array with variable transmittance. This provides switching between 2D and 3D display. As a disadvantage, though, the light has to penetrate two LCD panels, i.e. through a great number of devices as, e.g., polarizing filters, liquid crystal layers and other component parts such as carrier substrates, so that brightness is reduced in both 2D and 3D display.
In U.S. Pat. No. 6,157,424, a 2D/3D display is described in which two LCD panels are arranged one behind the other, one of which is used as a barrier that can be switched on in addition.
WO 02/35277 describes a 3D display provided with a substrate that contains stripes having a first set of optical properties and, in between them, stripes having a second set of optical properties, and a polarizer. Among other things, this permits switching between 2D and 3D by means of rotating the polarization, or adding or removing a polarizer.
Another display switchable between 2D and 3D is known from U.S. Pat. No. 6,337,721. This device is provided with several light sources, a lenticular, and at least one functionally essential, switchable diffusion disk. These components provide various illumination modes for obtaining a 2D or 3D display.
Known from U.S. Pat. No. 5,897,184 is an autostereoscopic display with a reduced-thickness illumination module for portable computer systems, which allows switching from 3D to 2D and vice versa in a zone-by-zone mode. The downside is that it is a two-channel 3D display for a single viewer, who has to occupy a fixed viewing position at that. Moreover, the brightness of the image in the 3D mode is lower than that of comparable two-channel 3D displays. This refers to such 3D displays that present exactly one left and one right image. Furthermore, a viewer who is not in the correct viewing position depthwise in front of the 3D display will see disturbingly strong moiré effects. In the 2D mode, the light available for the 3D mode is dispersed, among other means, with the aim to undo the 3D image separation by homogenizing the illumination. In arrangements with switchable diffusion disk, this results in a reduction of image brightness in the 2D mode, because the dispersive state of such diffusion disks has a transmittance of less than 1 (for example, approximately. 50%). Besides, manufacturing the device takes much effort and cost. As a further disadvantage, the insertion of a switchable diffusion disk increases the distance between the illuminating module and the image-reproducing panel, which inhibits normal viewing distances especially in 3D displays with small pixels and/or high resolution.
U.S. Pat. No. 5,134,345 describes an illuminating system for high-resolution and 3D displays, which, at first, generates certain illuminating patterns in a time-sequence (stroboscopic) mode. Another embodiment is a 2D/3D display provided with a diffusion disk that can be switched between a transparent and a dispersive mode, the latter being used for the 2D mode.
U.S. Pat. No. 5,500,765 describes a way to cancel out the effect of a lenticular by hinging down on it a complementary arrangement of lenses. This virtually switches off the 3D display. This approach works only with lenticular systems and requires the fabrication of an exactly complementary arrangement of lenses. Other drawbacks are sensitivity to dust, and increased reflection losses.
DE 100 53 868 C2 describes an arrangement allowing the choice of 2D or 3D display, which uses two light sources, with the 3D illumination always being switched off, or its light blocked, for the purpose of 2D display. The disadvantage of this arrangement is that the luminance of the 2D illumination cannot be made sufficiently homogeneous. Moreover, if a commercial light guide is used for 2d illumination, its macroscopic structure is visible to the viewer or viewers, as a rule, creating a disturbing pattern, whereas an invisible microscopic structure is difficult and expensive to manufacture.
The purpose of JP 10268805 it to generate a bright 2D image and ensure equal brightness for the 2D and 3D displays. This is attempted by using a lenticular screen as an illumination barrier arranged behind an image display means. In addition, a weakly diffusing disk is arranged there for temporary cancellation of the effect of the lenticular.
The disadvantage of this arrangement is that it requires an inherent light source for light of parallel rays, so that, in strict sense, there is no 3D viewing space but only a single, fixed 3D viewing position. Moreover, parallel radiation requires a complicated light guide in the “side light mode” used in this arrangement. A complicated, expensive “side light” would also be required for adding a “parallelization structure” to the light guide face opposite the outcoupling face, i.e. the one facing the viewer. Because of the optical lenticular screen method, the foci, e.g., in case of oblique parallel illumination would not lie in one plane of the diffuser. With 3D display, this would give rise to varied degrees of image blurring, especially when viewed obliquely.
US 2003/0011884 A1 provides for switching between 3D and 2D with “diffusing means”. Compared with a purely 3D display, the 3D/2D display contains additional “converting means”, which consist, in “the second condition” (meaning the 2D mode), “diffusing means” intended to provide 2D display in various ways.
The downside of this arrangement is that resolution in the 2D mode is very poor and “full resolution” cannot be reached in that mode. As a result, for example, text presented in the 2D mode is illegible. In the arrangements illustrated in FIG. 9 and FIG. 10 of US 2003/0011884 A1, which feature a switchable diffusing layer 94 inside the lenticular screen 15, the optical distance between the diffusing layer and the subpixels may be smaller, but still is relatively large. In addition, the manufacture of such a lenticular screen is difficult and expensive, and because of the added switchable diffusion properties it has further drawbacks. Further, it does not reach the ambient light compatibility of conventional 2D displays.
A lenticular screen is also preferably used in WO 99/44091. Here, the image-separating lenticular screen is intended to act as a “light diffusing” component when brought into a position close to the image display means. The lenticular screen itself is not made with light diffusing properties, neither on its convex or its plane surface, nor in its interior. The diffusing effect is intended to be brought about by the lenticular grid itself. This means, however, that the diffusion layer has a finite distance from the image display means and a distance of virtually 0 mm from the image separator. Therefore, the diffusing layer must necessarily impair the 2D image on the image display means and cannot cancel the image-separating effect of the lenticular screen. As a result, text presented in the 2D mode with these arrangements remains illegible; furthermore, they do not reach the ambient light compatibility of conventional 2D displays.